The present invention relates to a radiotelephone communications system and more particularly to a communications technique between a regional radio transmitter/receiver and a regional switching center in such a system whereby transmission efficiency may be increased and transmission costs reduced while at the same time simplifying the handoff process.
Conventionally, radiotelephone networks have been analog in nature, system channels being defined by separate radio frequencies upon which speech information is impressed using, for example, frequency modulation. In recent years, however, attention has shifted toward digital radiotelephone networks. The main advantage of a digital implementation is more efficient utilization of the frequency spectrum by using the same frequency for more than one call (TDMA). Other advantages of a digital implementation include robustness of communications, lower cost and power consumption, and lighter more compact equipment. In Europe, a separate radiotelephone network is being implemented for digital users. In the United States, however, it has been determined that radiotelephone networks shall be both analog and digital-capable.
The different nature of analog and digital communications gives rise to special problems in hybrid systems required to handle both. Under certain circumstances, it may become necessary to change a user from a digital radio channel to an analog radio channel without changing regional radio transmitter/receivers. Changing channels is known as "handoff". For example, since digital radio channels are more susceptible to time dispersion than analog radio channels, if the time dispersion on the digital radio channel becomes too great, and no other suitable digital radio channel is available, it may be necessary to handoff the user from a digital radio channel to an analog radio channel. Such handoffs create special problems for communications between the regional radio transmitter/receiver and the regional switching center.